1. Field of the Invention
The present invention relates to a battery charger suited or charging a battery, such as a nickel metal hydride battery, which emits high heat while being charged.
2. Description of the Related Art
Presently, a chargeable battery which can be repeatedly used for the power supply of, for example, a power tool is used. A nickel cadmium battery is popular as a battery for the power tool, and a battery charger for quickly charging a battery by applying high current to the battery is used. Specifically, the battery is quickly charged in about 20 minutes and a power tool can be continuously used by changing a battery to a battery which has been charged.
The inventor of the present invention studied improving the performance of a power tool by using a nickel metal hydride battery as a battery therefore. Although the nickel metal hydride battery can increase a capacity compared to a nickel cadmium battery, it generates high heat while being charged. If the temperature of the battery becomes high by the generated heat, the electrodes and separator of the cell within the battery deteriorate and battery life is shortened. Due to this, it is impossible to quickly charge the nickel metal hydride battery with high current as done for the nickel cadmium battery stated above.
Considering this, the inventor of the present invention contrived charging a battery while changing charging current based on a map in which the absolute temperatures of the battery and temperature rise values are mapped and detecting completion of battery charge from the map. However, if charging a battery by using this method, the switching of current values occurs frequently even in the earlier and medium periods of battery charge and charging time is disadvantageously lengthened. That is, the states of batteries to be charged vary such as one which capacity is almost zero and one which capacity remains almost full. Here, it is necessary to create the map as the greatest common measure of various states. In case of charging the above-mentioned batteries in various states or, for instance, charging a battery which capacity is almost zero in accordance with the map, current is switched to low current (such as 3C) based on detected temperature rise values even while the maximum current capacity (such as 4C) of the battery charger can be applied to the battery, thereby making time longer for completing battery charge. Further, in case of determining charge completion based on the map, it sometimes occurs that charge completion is determined after charging capacity exceeds 100% depending on the conditions.
Moreover, in case of obtaining charging current by retrieving a map from the absolute temperature of the battery and the temperature rise value, it is required that the battery temperature rise value, i.e., the accuracy of a temperature differential value is high so as to obtain an appropriate charging current value. In other words, if the accuracy of detecting the temperature differential value is low, an appropriate temperature value is not selected to thereby cause unnecessary switching of charging current as stated above. In that case, however, as temperature resolution increases, cost increases. It is, therefore, needed that unnecessary switching of current values can be prevented without increasing temperature resolution.